I never really checked how my back looked during a squat in the past, but recently I did because I was curious, and noticed my lower back curves when my ass gets to the grass. I've read it's because of tight hips and hamstrings. For about the last couple months I've done a lot of stretching (not just for better squats, more so for general health), but it's still curving. I've also tried to widen my stance, but no luck.

Pretty much everything I've read has said that 'ass to grass' is unnecessary, and to squat down so that the thighs are just below parallel to the floor. Like the others above, the back rounds when the load is too heavy. Ironically, you can probably lift more if you go down to just below parallel.